1. Field of the Invention
The present invention relates to a disc-driving apparatus and a method for controlling the same that could well be applied to an optical disc-driving apparatus, which has plural objective lenses and a light source producing lights having plural wavelengths, to thereby enable multi-wavelength accommodating recording and reproduction alone.
2. Description of Related Art Recently, an optical disc-driving apparatus has often been used which records an informational signal into a disc-shaped recording medium and reproduces the informational signal from it. Such an optical disc-driving apparatus is equipped with an optical pickup that moves in a radial direction of a disc-shaped recording medium mounted on a disc table to irradiate this disc-shaped recording medium with a laser beam through an objective lens, thereby recording or reproducing an informational signal.
The optical pickup is adjusted for proper focusing by detecting a focusing error signal and displacing the objective lenses based thereon in a direction (focusing direction) tangent to a recording face of a disc-shaped recording medium. Besides, tracking adjustment is performed by detecting a tracking error signal and displacing the objective lenses based thereon in a roughly radial direction (tracking direction) of the disc-shaped recording medium. The tracking error signal is detected by, for example, a differential push-pull (DPP) method.
As for this type of differential push-pull method, Japanese Patent Application Publication No. Hei 3-012830 has disclosed a tracking error detection method. According to this detection method, by arranging a pair of side beams for detection of a tracking error signal on the same side behind or ahead of a main beam in a tracking direction, a difference between them is detected to thereby detect an error. By configuring such a detection method, a tracking error signal can be accurately detected not only in reproduction but also in recording and erasure.
Further, as for detection of a tracking error signal, Japanese Patent Application Publication No. Hei 5-012700 has disclosed an optical head unit. According to this optical head unit, spots of respective two sub-beams have been arranged behind and ahead of a spot of a main beam on an optical disc in such a manner that they may be respectively placed at different edges of a track in which the main beam is displaced. Reflected lights from the two sub-beam spots are used to detect a tracking error signal. By thus configuring an optical head unit, the tracking error signal will encounter no offset at a boundary between a recorded portion and an unrecorded portion of the optical disc, so that tracks can be followed stably.
Furthermore, as for detection of a tracking error signal, Japanese Patent Application Publication No. Hei 6-236567 has disclosed a tracking error detection device and an optical disc apparatus equipped with it. This tracking error detection device has a diffraction grating(s) for generating high-order diffracted light, to irradiate a surface of an optical disc with a total of five optical spots of one main spot plus four side spots and generate two types of tracking error signals from two suites of the side spots so that inter-spot spacing distance in a radial direction of the disc between the suites of the side spots may be set to each inter-track space. By thus configuring a tracking error detection device, it is possible to obtain a good tacking error signal for a plurality of types of optical discs having a large difference in inter-track space.